In designing and handling mechanical structures, one commonly encounters situations in which the measurement of high frequency vibrations plays an important role. Such high frequency measurements are valuable especially in the area of structural health monitoring. While fiber Bragg gratings (FBGs) have been well studied and understood for quasi-static strain and temperature measurements, their use for high frequency, dynamic measurements were impeded by sampling frequency limited optical spectrum analyzer components. Multiple intensity modulation-based methods have allowed high frequency interrogation of FBGs. The use of intensity modulation for the interrogation of FBGs involves the use of an optical filter that can adjust the intensity of incoming light based on the wavelength of the light. For example, some have designed a high rate FBG interrogator through the use of a fiber Fabry-Perot tunable filter. The filter uniquely converted wavelengths into a fixed level of intensity. Others have used an asymmetric FBG as an optical filter that preceded a single photodetector. The method was used to demonstrate measurement of signals up to 50 kHz. Another intensity based FBG interrogation method uses an arrayed waveguide grating (AWG). The AWG, which consists of an array of narrow-band optical filters, allow dense multiplexing of FBGs (theoretically up to 128 FBGs). Another method makes use of an extremely narrow band laser source and a regular FBG for the dynamic detection of high frequency events. By tuning the laser source at the spectral full width at half maximum (FWHM) of a highly apodized FBG, any wavelength shifts of the FBG will be translated to intensity fluctuations in the reflected spectrum. Such system demonstrated the measurement of ultrasonic vibrations. Others have performed high frequency FBG interrogation using a similarly narrow band laser source. The above methods are suitable for use in structural health monitoring, and are gaining popularity for use in cases where dynamic signals can point to structural damage. High frequency FBG interrogation methods utilizing similar principles as in the above were used to analyze the effects of impact on various structures, as well as for monitoring of structural changes over time (e.g. delamination).
The intensity modulation methods described above require the use of special filters or laser sources. A separate class of such methods exists in which regular FBGs can be used as the filter, thus potentially driving down costs and simplifying the system. For instance, some have proposed similar set ups in which the signals of a sensing FBG could be measured without the use of additional filtering or interrogation techniques. In such methods, the reflection spectrum of two wavelength matched FBGs are compared by a photodiode. The more overlap there is between the center wavelengths of the two FBGs correlate, the higher the amount of power received by the photodiode. If the FBGs are placed adjacent to each other at close proximity, the effects of thermally induced strain can be minimized. These wavelength-matched methods utilize dual FBGs per sensor to free the user from the cost and sampling frequency related limitations of specialized components, such as scanning optical filters, but may need additional equipment for multiplexing (e.g. switches).
In the following dynamic FBG interrogation system and method, a wavelength matched set up using regular FBGs is described that is also able to measure dynamic signals such as acoustic vibrations. The dynamic FBG interrogation system and method utilizes the transmission spectrum of one FBG and the reflection spectrum of the other. The benefit of this approach is the inclusion of fewer optical couplers, which can be a source of optical power loss in other systems, and as will be seen can thus increase the sensing range of the system.